|C NAME and Number
|SIZE IN MM (mm)
|STANDARD PAPER SIZE
|917 x 1297
|648 x 917
|458 x 648
|324 x 458
|229 x 324
|A4 sheet, A3 sheet folded in half
|162 x 229
|A5 sheet, A4 sheet folded in half
|114 x 162
|A6 sheet, A4 sheet folded in four equal parts
|81 x 113
|A7 sheet, A5 sheet folded in four equal parts
|57 x 81
|40 x 57
|28 x 40
A quick and easy way to remember which envelope size you need is to swap the "A" letter of your paper size for the letter "C". For example, if you want an envelope to fit an A4 sheet of paper, then you'll need a C4 envelope (and so on).
Paper comes in A and B sizes. The C envelope size is the geometric mean of the A and B size of the same number. This calculation produces a size between the A and B dimensions to make an envelope to perfectly hold the A size paper. For example, if you calculate the geometric mean of A4 and B4 paper this will give you the dimensions of a C4 envelope (which is the ideal size to hold an A4 sheet of paper unfolded).
Yes, a C4 envelopes is larger than a C5 envelope. A C5 envelope is designed to fit A5 paper and a C4 envelope is designed to fit A4 paper. The ISO standard paper sizing system works by using A0 size paper as the base point and largest size. The number next to the A specifies the number of folds in half needed from an A0. For example, A2 paper is A0 folded twice, A3 paper is A0 folded 3 times, A4 paper is A0 folded 4 times. Therefore, the number increases as it relates to the number of folds.
The DL envelope originated in Germany in the 1920's and was then known as DIN Lang, however these days DL is known to stand for 'Dimension Lengthwise”. A DL envelope is designed to fit an A4 piece of paper, folded into thirds. This is one of the most common envelope sizes for business letters and you will find much of your standard post arrives in a DL size envelope.